Reasons for mechanical seal failure

Thread Content

1. Leakage during static installation testing. After the mechanical seal is installed and adjusted, a static test is generally carried out to observe the leakage rate. If the leakage amount is small, it is usually due to a problem with the seal ring of the rotating ring or the stationary ring ； A large leakage amount indicates a problem in the friction pair between the rotating and stationary rings. Based on the initial assessment of the leakage amount and identification of the leakage location, manual rotation of the shaft is carried out for further inspection; if there is no significant change in the leakage amount, then there is a problem with the static or dynamic ring seals ； If there is a significant change in the leakage amount during cranking, it can be concluded that there is a problem with the friction pair between the rotating and stationary rings ； If the leaking medium is ejected axially, there are usually problems with the rotating ring seal; if the leaking medium sprays in all directions or leaks out through the water cooling holes, it is often due to a failure of the stationary ring seal. Furthermore, leakage channels can also exist simultaneously, but there is usually a primary and secondary distinction; with careful observation and familiarity with the structure, it is definitely possible to make the correct judgment. 2. Leaks that occur during trial operation. After a static test, the mechanical seal used in pumps prevents leakage of the medium due to the centrifugal force generated by high-speed rotation during operation. Therefore, during commissioning, mechanical seal leakage, after ruling out failures in the shaft-separation and end-cap seals, is basically caused by damage to the friction pair of the rotating and stationary rings. The main factors causing seal failure in friction pairs are: (1) During operation, abnormal conditions such as evacuation, cavitation, and pressure buildup generate significant axial forces, leading to the separation of the contact surfaces between the moving and stationary rings ； (2) Excessive compression during the installation of the mechanical seal led to severe wear and scarring of the end faces of the friction pairs ； (3) The rotating ring seal is too tight, preventing the spring from adjusting the axial movement of the rotating ring ； (4) The static ring seal is too loose; when the dynamic ring moves axially, the static ring detaches from its seat ； (5) There are particulate substances in the working medium, which enter the friction pair during operation, affecting the sealing surfaces of the moving and stationary rings used for flaw detection ； (6) Incorrect design selection, low specific pressure at the sealing surface, or high cold shrinkage of the sealing material, etc. The above phenomenon occurs frequently during trial operation; sometimes it can be eliminated by making appropriate adjustments to the stationary ring seat, but in most cases it is necessary to disassemble and reassemble the components and replace the seals. Failure resulting from the loss of the lubrication film at the two sealing surfaces: a) dry friction occurs when the pump is started with no liquid in the sealing chamber due to the sealing load on the surfaces ； b) The medium’s pressure is below the saturated vapor pressure, causing the liquid film on the end faces to flash and lose its lubricating effect ； c) If the medium is a volatile substance, when scaling or blockage occurs in the mechanical seal cooling system, the friction at the end faces and the heat generated by the rotating elements stirring the liquid cause the saturated vapor pressure of the medium to rise, which also results in the medium pressure being lower than its saturated vapor pressure. Mechanical seal failure caused by corrosion: a) pitting on the sealing surface, even penetration. b) Due to the welding of the tungsten carbide ring to the stainless steel seat, intergranular corrosion is likely to occur in the stainless steel seat during use ； c) Welded metal bellows, springs, etc. are prone to rupture under the combined effect of stress and medium corrosion. Mechanical seal failure caused by high-temperature effects: a) Thermal cracking is the most common failure phenomenon in high-temperature oil pumps, such as sludge pumps, re-refining oil pumps, and bottom pump in atmospheric and vacuum distillation towers. Radial cracks in the ring surface can occur at the sealing surface due to dry friction, sudden interruption of cooling water, intrusion of impurities into the sealing surface, evacuation, and other such conditions ； b) Graphite carbonization is one of the main causes of sealing failure when carbon-graphite rings are used. During use, if the graphite ring exceeds its allowable temperature (usually between -105 and 250°C), resin will precipitate on its surface; the resin near the friction surface will carbonize. In the presence of a binder, this will cause the material to foam and soften, thereby increasing leakage at the sealing surface and leading to seal failure ； c) Auxiliary seals (such as fluororubber, EPDM, all-rubber types) will rapidly age, crack, and harden and lose their elasticity when exposed to temperatures above the allowable limit. The flexible graphite currently in use has good high-temperature resistance and corrosion resistance, but its resilience is poor. It is also prone to cracking and can be easily damaged during installation. Sealing failure caused by wear of the sealed end face: a) Factors such as poor wear resistance of the materials used in the friction pair, high friction coefficient, and excessive end face pressure (including spring pressure) can all reduce the service life of mechanical seals. For commonly used materials, in order of wear resistance: silicon carbide—carbon graphite, cemented carbide—carbon graphite, ceramics—carbon graphite, sprayed ceramics—carbon graphite, silicon nitride ceramics—carbon graphite, high-speed steel—carbon graphite, surfacing with cemented carbide—carbon graphite. b) For media containing solid particles, the penetration of solid particles into the sealing surface is the main cause of seal failure. Solid particles that enter the end faces of the friction pair act as abrasives, causing severe wear on the seal and leading to its failure. A proper gap in the sealing surface, the degree of balance of the mechanical seal, as well as the flashing of the liquid film on the sealing surface, are all major causes of the sealing surface opening and allowing solid particles to enter. c) The balance degree β of the mechanical seal also affects the wear of the seal. Under normal circumstances, a balance level of around β=75% is most suitable. β